Electric resistance element



'18 I in atmospheresi'which. changedsin character; such S practice Omanindustrialvscaler,

Patented Jan. 30; 1940 UNITED STATES v Z,188,399 A NT-OFFICE 1 pa sam QELECTRIC RESISTANCE ELEMENT I Clarence George Bieber; Huntington; W; Vaas signer to The lnternationaliNickel Company,

'. Inc., New York, any" Bio Drawing;

'10 ticularly forv electrics-resistance; elements intended to operate:at elevated -.temb ratures:; especially above, 180091; IQLQQIP;YI}1QY1,OI xlreasonsginclud ing their suscep'tibiliitoazembrittlement"orioxidation .at such: high temperatures;particularly as from oxidizing to reducing; 1 -Althgu gh; many proposalshave been :made; none asfar: as 21am aware, has been; whollysuccessfuland; satisfactory commercially,: especially when *{carried :into' aAn-obiect tor the :inven on :is to; Davide's electric resistanceelement: comprising 2a: heat-- -.-treated' chromium "plated nickel or;nickel alloy wire which can be operatediin, oxidizingror ire ducingatmospheres at elevated temperatures in cluding 1800-2500 F. 21:

A further object of the invention is-to provide an electric resistance.elemefltutilizingthe electrical resistance properties; of nickel cr'ianickel nickel duertoloiddationrma, 1. V It is also within.thelcontemplationofthe inventionto provide a processfor heat treatingchromium ,plate'd nickel or nickel alloy:.-wire whereby an 'electric'resistance element can be produced in a suitable condition-for usein'electric resistance furnaces. V a 1 Other objects rand advantagesoithe invention will becomeapparent from thefollowing description of apreferred embodiment of the and below '-'2600 ;F. iwithout.destructionipf. the

invention and a procedure for-carrying the invention into practice. 1

According to the present and inexpensive resistance heating element isprovidedby utilizing the desirable features of nickel and' nickel'alloysandavoiding the shortcomings thereof. 1 have discovered that'envelopmentof nickel or nickel alloy with chromium, especially the electrodepositedtype, and the subsequent formation, of a bond of a nickelchromium alloyprovides an electric resistance element with adesirable temperaturecoefficient, an excellentresistance to oxidation and emperatures andchanging atmospheres including oxidizing and reducing and with highmelting point. I a v A method of producing the new electric resistanceelement is to electrolytically a deposit Application Serial N0 1.109,307

alloy at elevated temperatures above 1800?- F.-

invention an emblembrittlement, especially at'super or elevated tern arelatively wmnmaw m Noe i; "(1936.

.thintla yers of chromium about; '.ooo2" to 'about 1 0.0Q5 'f:,-thick,upon nickel'or a nickel alloy wire :yorfgthe like.- Thesplated productissubiected to -aspecial-heat) treatment in a hydrogen Iat'mos pheret;It is .preferred to cefie'ct' the treatment by heating the wirerapidlyby the passage of an electric current threthrough; The heat i:i-treatmentEis such-as'to cause chromium to diffuse f a "-rilltofthesurface skin'or layer oi the nickel or nickel alloy so astoformranintermetallic alloy of chromium with thebase metalor alloy. The treated,wire: constitutes an improve ,electric resistance element which can beinstal ed in any of electric resistance heating furnace and willfunction satisfactorily ataj much higher wyotherfbase metals-or, alloysknown to the trade.

a abetter understanding: of the. invention, a

preferred embodimentand a preferred-procedure A oflmakingrthepresentilow,priced resistance eleg-ment :willibe' described;1

-: .;;Nickel wire of :any suitable diameter is pickled ree =fr'om-oxide,'. etc. and 1 carefully cleaned with ipg'removez anygrease;or,..dirt etc, It is preferred -'to:then:clean{thexwireelectrolytically in a so-'v avlutio'n containing 10 per cent of sodiumcar edroxider gThe wire acts as a cathode with a ,cleaned'further ascathode ina solution of about 50 'sulfuricfacid wi'tha current densityof 1about20 amperes per squareyfoot. The cleaned }wire isflnallyrrinsedwith water. The wire" is coated with about 0.00021 to about 0l005" ofchromium by the electrodepos'ition in a chromic i j [acid bathcontaining approximately 250 grams of the chromic acid and' a bout 2.5gramssulfuric acid perliter'using a current density of temperature "ofabout 1159-120". F.

After this plating operation; the 1 chromium plated nickel wire issubjected toheat treatment 10G1-b01i tetrachloride or sodiumtri-phosphat'e to approximately i; ampere per square inch and a- -Forthepurpose of giving those skilled infthe bonate and about- 1 per centoff'sodiumhy- .ccurrent densityofabout m amperes per square" Q g .foot.Following this treatment," the wire is by passing a regulated electriccurrent there- Y through sofas to maintain the wiretemperature 1 atabout 1800 to about 2000 E. for about 1' to about? hours'in a hydrogenatmosphere which I have found to be best suited for the diilusion oralloying of the chromiumwith the nickel; The 7 product is, then anelectric resistance v heating an electrical resistance furnace. Testshave demonstrated that it will operate satisfactorily without oxidationat a higher temperature than any base-metal resistor, is more convenientto use -than non-metallic resistors {of the 'carborunf dum type, and ischeaper to the public.

element capable of being used for any kind of l5 t,v.; temperature;.than Jprior: electric resistance "eleg mentsacomposed bfchromium-nickel. alloy or In carrying the invention into practice, theimproved electric resistance element was subjected to severe tests todemonstrate its usefulness and to establish its unique properties. Someof these tests will now be described.

A rectangular furnace with a heating chamber 5" x 8" x 14" was providedwith a hair-pin type of the improved electric resistance heating elementmade in accordance with the procedure outlined hereinabove byelectro-depositingabout 0.001 inch of chromium upon a nickel wire aboutT g" in diameter and heat treating the same at about 1800 to about 2000F. .in hydrogen for about 2 hours to alloy the chromium with the nickel.This furnace has been used.intermittently for 40 heats of 8 hours eachat temperatures of 2000-2300 F. in an oxidizing atmosphere, and for 8heats of Shows each at temperatures of 2000-2100 F. in a hydrogenatmosphere. practically as good a condition asat the time ofinstallation.

A cylindrical furnace about 6" in diameter and 10" high was providedwith a helical unit constituted of the present electric resistanceelement of diameter nickel wire plated with about 0.001 inch of chromiumafter heat treatment in hydrogen for about two hours at'about 1800 toabout 2000 F.- This furnace has been used for 50 heats of about 4-8hours each at temperatures of about 2150 to about 2350 F. in

a hydrogen atmosphere and for 12" heats of about 8 hours. each attemperatures of about The heating unit was found to be in (b) Reheatedto 2000" for 8 hours and cooled over-night, r

(c) Reheated to 2100 for 8 hours and cooled over-night,

(d) Reheated to 2200 for .8 hours and cooled over-night,

(e) Reheated to 2300 for 8 hours and cooled over-night,

(f) Reheated to 2500 F. for and showed that the nickel interior hadactually been in a molten condition inside the chromium shell, and thatthe wire had operated for a time as a molten resistor.

It'is to be observed that the presentapplication provides an improvedelectric resistance heating element which possesses new and uniqueproperties and superior characteristics to prior electric resistanceheating elements. Some of the new, improved and unique properties and2000 to about 2100" F. in an oxidizing atmoscharacteristics may beappreciated from the following schedule:

Schedule l V V Prior alloy of 607 New chrome Prior silo of 80% Prioralloy of 85% 1' 257 Fe, 117 plated nickel Ni, 20 Cr Ni, 15% Gr Cn40% Mn0 Resistance .Q l6 microhms per 100 microhms 85 microhms per 105microhms per 1 cm per cm. cm cm Temperature resistance co-efiiclent 002per F .00006 per "F 00008 per "F .0001 per l Maximum usefultemperatures... 2600 F 2000 F 1900 F 1400" F. Resistant to atmospheresExcellent. Good Fair Poor.

. do ;.do Good Resistant to fluctuation Fair.

phere. The improved heating unit or' electric resistance element is inpractically as good a condition as when it was originally installed.

Careful examination of the improved electric resistance elements in bothfurnaces described hereinabove showed practically no signs ofdeterioration, their electrical characteristics "have not changed and apractically constant electric current has continued to give the samefurnace temperatures at all times. The remarkable 'efliciency of thenovel electric resistance element can be judged further by comparisonwith the fact that a A" diameter conventional heating unit made from analloy containing approximately per cent of chromium and about 80 percent of nickel was used'in the cylindrical furnace and was found to havefailed due principally to oxidation after three heats of about 4 hourseach at about 2300'F. in a hydrogen atmosphere.

A further description of my new results will show the remarkableperformance at elevated temperatures of my improved resistance element.Some 0.051" nickel wire with. a heat treated 0.001 inch chromium coatingwas wound on a refractory cylinder and assembled into a tube furnace.This furnace was subjected to the fol lowing heating cycles in air:

(a) Heated to 1900 F for 8 hours andcooled' over-night,

It is also to be observed that the new and improved electrical heatingresistance element has a longer life at temperatures of 2000" F. andabove. The life ofthe present improved electrical resistance heatingelement is distinctly superior to the life obtainable with priorelectric resistance heating elements as may be clearly seen'from thefollowing table:

'Table Life at Newyctc. Prior. etc. ggy

rnfiiiim infiifiia iti it,

2i00F V. T T Indefinite 1000 57 -l7 2200F 9e0 V 22 11 ,5

N0TE. Indeflnite means lonucr than 1080 hours or 10 weeks."

Furthermore the present invention has been applied to a nickel-chromiumwire which was coated with a film of nickel having a thickness of about0.00025 to about 0.0005" and then plated with afilm of chromium having athickness of about 00002 to about 0.0005 This doubly plated wire hadincreased life at temperatures This doubly plated wire is very'usefulfor operating'at high temperatures. A satisfactory composition of thenickel-chromium alloy was found to be 50% to about 90% of nickel, 10% toabout 25% of chromium, .l% to about 25% of iron and especially 80% ofnickel, 19.8% of chromium and .2% of iron.

Other advantages of my improved resistance wire will become readilyapparent to those familiar with the subject. For instance, I couldchromium plate a chromium-nickel resistance wire and increase itsoperating range to higher temperatures, but I prefer to provide a lessexpensive article of manufacture by utilizing nickel, or some of itscommercial alloys, as a base product. r

Although the present invention has been described in connection withpreferred embodimentsit will be appreciated that variations andmodifications may be resorted to without departing from the spirit andscope of the invention. For instance nickel alloys may be used. Thusnickel alloys having the following composition may be used:

I The size of the wire or element may vary depending upon specificconditions. For practical purposes the size may vary from 0.005" toabout 1" in diameter or thickness.

ventional types, such as rounds, flats, strip, ribbon or T grid.Moreover tungsten or molybdenum may function like chromium but are notascommercial as chromium. All of these variations and modifications are tobe understoodto be within the purview of the present specification andthe scope of the appended claims.

What I claim is:

' 1. As a new article of manufacture, a composite electrical resistanceheating unit comprising a core constituted of a nickel-chromium alloy,an outer layer of chromium enveloping said core, an intermediate layerof nickel, and diffusion zones between the outer layer and the nickellayer and between .the nickel layer and the core intimately bonding thelayers with each other and with the corejsaid chromium layer beingsubstantially more resistant to failure at temperatures in excess of2200" F. than said core.

'2. As a new article of manufacture, a composite electrical resistanceheating unit comprising a core containing a nickel-chromium-iron alloy,an intermediate layer of nickel surrounding said core, an outer layerofat least one metal of the group consisting of chromium, molybdenum andtungsten surrounding said nickel layer, and diffusion zonesbetween theouter layer and the nickel layer and between the nickel layer and thecore intimately. bonding the layers with each other and with the core,said outer layer being substantially more resistant to failure attemperatures in excess of 2200 F. than said core.

CLARENCE GEORGE BIEBER.

The heating element may have various shapes including the con-

